1. Technical Field
The present invention relates to an all-optical frequency upconversion method and an all-optical frequency upconverter in a radio-over-fiber system, and in particular, to all-optical frequency upconversion method and an all-optical frequency upconverter in a radio-over-fiber system that mix an optical IF (Intermediate Frequency) signal with an optical LO (Local Oscillation) signal to generate an optical RF (Radio Frequency) signal.
2. Background Art
Recently, in the fields of wireless communications, studies for improving speed and quality of broadband multimedia traffic are actively in progress. In a broadband multimedia service, for massive data transmission, the frequency of a radio signal is being increased up to the microwave or millimeter-wave band. As the frequency is increased, an electric transmission line, generally used for transmission of high-frequency electrical signals, experiences a high transmission loss. A radio-over-fiber (RoF) technology has been proposed so as to solve the problem arising from the high transmission loss of the electrical transmission line. According to the RoF technology, an optical RF signal containing a microwave or millimeter-wave radio signal is transmitted through an optical fiber, which has features including ultra-wide broadband and low transmission loss.
The RoF system has attracted much attention in the broadband wireless access fields, such as wireless backbone network, remote medical service, and multimedia broadcasting. With the RoF technology, a cost-effective broadband convergence network incorporating a high-speed optical communication network and a wireless communication network can be constructed.
FIG. 1 is a conceptual diagram showing a general RoF system.
Referring to FIG. 1, the RoF system includes a central station 10, a remote node 30, and a remote antenna station (RAS) 40.
Data (carried on an IF signal) from the high-speed backbone network is modulated by a modulator at the remote node 30 using an LO signal to produce an optical RF signal 31. The optical RF signal 31 is then transmitted to the RAS 40 through an optical fiber 32, which has features including ultra-wide broadband and low transmission loss. The RAS 40 converts the optical RF signal 31 into a radio signal and sends the converted radio signal through an antenna.
Methods for converting data into the optical RF signal 31 include one that uses an electro-optic modulator and the other that uses all-optical frequency converter.
The all-optical frequency conversion method is suitable for an ultra-wide broadband RoF system because the wavelength division multiplexing (WDM) system, which is capable of markedly improving the frequency utilization, can be used. The WDM system is a technology that multiplexes multiple channels on a single optical fiber by using different wavelengths. That is, the channels are allocated to different wavelengths spaced at an equal wavelength interval, then the signals are correspondingly carried on the channels, and subsequently the channels are optically multiplexed. Accordingly, different signals are transmitted through a single optical fiber.
The all-optical frequency conversion system reported previously uses XGM (Cross-Gain Modulation) or XPM (Cross-Phase Modulation) effect inherent in a semiconductor optical amplifier (SOA). The all-optical frequency conversion system employing the XGM or XPM effect of the SOA is advantageously applied to the WDM system due to its wide LO frequency bandwidth characteristics. However, it has a limited IF frequency bandwidth (usually limited to several GHz or less) arising from the carrier lifetime of the SOA.
The limited IF frequency bandwidth characteristics of the all-optical frequency conversion system employing the XGM or XPM effect of the SOA adversely affects application of the sub-carrier multiplexing (SCM) system, which is also capable of markedly increasing the data transmission capacity, to the RoF system, together with the WDM system.
Accordingly, for implementation of the millimeter-wave ultra-wide broadband RoF, there is a need for a novel all-optical frequency conversion system that is capable of simultaneously applying the WDM system and the SCM system, and has wide LO and IF frequency bandwidths.
There are reports on all-optical frequency conversion system that use four wave mixing (FWM) effect by a HNL-DSF (High-nonlinear dispersion shifted fiber) and a Raman pump (“Seamless Integration of an 8×2.5 Gb/s WDM-PON and Radio-Over-Fiber Using All-Optical Up-Conversion Based on Raman-Assisted FWM”, IEEE Photon. Technol. Lett., vol. 17, pp. 1986-1988, September 2005, Jianjun Yu, Jinxing Gu, Xiang Liu, Zhensheng Jia, and Gee-Kung Chang). These all-optical frequency conversion systems utilizing the FWM have wider IF frequency bandwidth than the all-optical frequency conversion system utilizing the XGM or XPM effect of the SOA, and thus the WDM system and the SCM system can be simultaneously applied to increase the bandwidth of the RoF system. However, in the known all-optical frequency conversion system, which uses the FWM effect by the HNL-DSF and the Raman pump, a no-gain optical fiber is used to use the FWM effect. For this reason, the HNL-DSF and the Raman pump having good nonlinearity are used to increase the conversion efficiency in the no-gain optical fiber. As a result, the length of the optical fiber to be used reaches approximately 1 km, and the Raman pump needs to be provided, which causes the complexity of the overall system configuration.